As a next generation power semiconductor device material, silicon carbide (which may be hereinafter also denoted as SiC) attracts attention. SiC has about 10 times the breakdown field strength and about three times the thermal conductivity of silicon (which may be hereinafter also denoted as Si), and SiC can achieve a power semiconductor device capable of operating at a high temperature with a low loss, which cannot be achieved with a Si power device.
For example, a high-breakdown voltage power MOSFET has a low ON-resistance and high breakdown voltage, and can achieve fast switching operation. Accordingly, it is widely used as a switching device for a power circuit such as a switching power supply. The device structure of the high-breakdown voltage power MOSFET has a vertical-type MOSFET structure in which a source electrode, a gate electrode, and a well electrode are formed on a substrate surface, and a drain electrode is formed on a back surface of the substrate. Double Implantation MOSFET (which may be hereinafter also denoted as DIMOSFET) structure in which a channel formation region (well region) and a source region are respectively formed on a substrate surface using ion implantation is an advantageous device structure in which the channel region can be easily formed with high precision, and this is also suitable for parallel operation.
When a DIMOSFET using a SiC substrate is formed, an electrode for connecting this device to an electrical circuit and the like is desired to be in ohmic contact. However, a generally used hexagonal single-crystal SiC substrate has 4H—SiC structure of which laminating cycle is 4, and an energy band gap thereof is 3.26 eV, i.e., three times the energy band gap of Si. Therefore, it is difficult to form ohmic contact with an electrode metal.
In contrast to the SiC substrate, a method for forming a nickel silicide layer in the electrode portion is known as a method for achieving a relatively low ohmic contact resistance. The nickel silicide layer is formed by depositing nickel on a SiC substrate surface of the contact region and thereafter performing thermal treatment. One of the problems in forming a nickel silicide electrode includes contact property degradation with the metal electrode due to nickel silicide surface precipitation of carbon (C).